Method for estimating the overturn risk of a vehicle

ABSTRACT

The invention relates to a method for redundantly estimating, i.e. by means of various sub-methods, the risk of lateral overturn of a vehicle. One sub-method involves estimating the risk of overturn according to the actual braking state of the vehicle—braking or strong braking, or not braking or slight braking—by monitoring the rotational speed behavior of the wheels on the inside of the turn. Another sub-method involves pre-calculating the rolling motion of the vehicle about the longitudinal axis thereof over a short time span of, for example, 0.5 s to 1.5 s on the basis of instantaneous movement parameters and assessing the risk of overturn on the basis of the anticipated rolling motion.

[0001] The present invention relates to a method of estimating theoverturn risk of a vehicle, according to the preamble of claim 1.

[0002] From German Patent Document DE 196 02 879 C1, a method ofdetecting the overturn risk of an ABS-equipped vehicle is known, duringwhich the transverse acceleration of the vehicle is constantlymonitored. When the transverse acceleration exceeds a defined limitvalue, a braking intervention takes place by means of a low testingbrake power. In this case, it is monitored whether the testing brakepower leads to an ABS intervention at the assigned wheel; that is, to alocking risk, which indicates that only a slight normal wheel forcestill exists or that the wheel has already lifted off the road and anoverturn risk is present. Thus, solely the start of the ABS controlintervention is used as an indication of an overturn risk.

[0003] It is an object of the invention to provide a method by means ofwhich the overturn risk can be estimated still more reliably.

[0004] This object is achieved by means of the characteristics ofclaim 1. Advantageous developments and further developments of theinvention are contained in the subclaims.

[0005] The basic principle of the invention consists of a method bywhich the risk of a lateral overturn of the vehicle can be estimatedredundantly, that is, by means of various submethods.

[0006] In the case of one submethod, the risk of overturn is estimatedas a function of the actual braking state of the vehicle—braking orstrong braking, or not braking or slight braking—by monitoring therotational wheel speed behavior of the wheels on the inside of the turn.

[0007] In the case of another submethod, the rolling motion of thevehicle about its longitudinal axis is precalculated over a short timespan of, for example, 0.5 s to 1.5 s on the basis of instantaneousmovement parameters and the risk of overturn is, assessed on the basisof the anticipated rolling motion.

[0008] The risk of a lateral overturn of the vehicle during corneringcan therefore be better estimated by the sequential or parallelimplementation of different monitoring methods.

[0009] For determining the risk of overturn, the transverse accelerationof the vehicle or the angular acceleration about the longitudinal axisof the vehicle is sensed in a continuous manner. The exceeding of adefined transverse acceleration limit value is a first indication thatthe vehicle is in a critical situation in which an overturn risk mayexist.

[0010] The method of examining whether an overturn risk actually existsis carried out as follows:

[0011] a) As a function of whether the vehicle is braked or not braked,when a defined transverse acceleration is exceeded, the brake pressureis changed at individual wheels and the occurring rotational wheel speedbehavior is monitored.

[0012] If the vehicle is not braked, individual or more wheels aresimultaneously acted upon by a low testing brake pressure. If, as aresult of the testing brake pressure, the rotational wheel speed doesnot change or changes only very little, this is an indication of asufficiently high normal wheel force on the road; that is, the wheel isnot in danger of lifting off the road. If, in contrast, the wheel isalready braked intensively by the slight testing brake pressure, this isan indication of a low normal wheel force or that the wheel has alreadylifted off the road.

[0013] If the vehicle is braked, the brake pressure at individual ormore wheels is simultaneously lowered and it is monitored whether therotational wheel speed changes little or greatly. If the rotationalwheel speed changes little or not at all, this is an indication that thewheel has already lifted off or is just about to lift off. If, incontrast, the wheel is relatively strongly accelerated, it can beconcluded that a sufficient normal wheel force still exists.

[0014] c) b)? In addition, by means of instantaneous movement parametersof the vehicle, the rolling motion is precalculated for a time span of,for example, 0.5 to 1.5 s. For this purpose, particularly the transversevehicle acceleration, the time gradient of the transverse vehicleacceleration as well as the frequency or the period of vibration of therolling motion of the vehicle about its longitudinal axis can be used.By means of defined motion equations, into which diverse vehicleparameters are entered, such as the mass of the unloaded vehicle, theloading condition, the position of the center of gravity of the vehicle,etc., the rolling motion to be expected and thus the risk of overturn ofthe vehicle can be estimated. The time span for which an estimation canbe carried out depends on the vehicle speed. The higher the vehiclespeed, the earlier a critical situation has to be recognized.

[0015] When a critical driving condition, that is, the overturn risk ofthe vehicle is recognized, a control intervention of the braking systemtakes place, for example, selectively at individual wheels, whereby theoverturn risk is reduced and the vehicle is stabilized. If a drivingsituation is estimated to be critical in a different fashion, thecontrol interventions considered to be required for stabilizing thedriving will therefore also differ. Preferably, the control interventionwith the highest braking demand will then be selected. In addition, theABS system of the trailer vehicle is permitted to reduce the selectedbraking demand, so that the vehicle motion can be controlled during thecornering.

[0016] A particularly advantageous field of application of the inventionis the commercial vehicle field because, specifically in this field,accidents occur repeatedly as a result of an overturn of towing vehiclesor trailer vehicles during cornering or during abrupt steering movementsin danger situations. The method is particularly suitable forsemitrailer units because dangerous driving conditions, which are causedby dynamic vehicle movements and would lead to the overturn of thesemitrailer, can be recognized more reliably and a braking interventioncan be carried out in time.

[0017] The method is preferably implemented in an electronic controlunit (ECU) which may be arranged on the towing vehicle or the trailervehicle or semitrailer. If the control unit is arranged on the towingvehicle, the control unit is connected by means of a connection linewith the ABS system of the trailer.

[0018] In the following, the invention will be explained in detail bymeans of an embodiment in connection with the drawing.

[0019]FIG. 1 is a flow chart of a method of estimating the overturn riskof an unbraked vehicle;

[0020]FIG. 2 is a flow chart for estimating the overturn risk of abraked vehicle; and

[0021]FIG. 3 is a flow chart for estimating the overturn risk on thebasis of instantaneous movement parameters of the vehicle.

[0022] The starting point for estimating the overturn risk of a vehicleis the determination of the transverse vehicle acceleration a_(trans),which can be achieved, for example, by means of a lateral accelerationsensor provided in the vehicle.

[0023]FIG. 1 is a flow chart for estimating the overturn risk of anunbraked vehicle, the transverse vehicle acceleration a_(trans) beingdetermined here in Step 1. “Unbraked” means that no braking demandsignal is present from the driver. In Step 2, the sensed transversevehicle acceleration a_(trans) is compared with a defined transverseacceleration limit value a_(trans max). If the sensed transverse vehicleacceleration a_(trans) is lower than or equal to the defined transverseacceleration limit value a_(trans max), it is assumed that an overturnrisk does not exist. If the vehicle continues to be unbraked, a jumptakes place back to Step 1.

[0024] If the sensed transverse vehicle acceleration a_(trans) is higherthan the defined transverse acceleration limit value a_(trans max), onat least one or more vehicle wheels on the inside of the turn, theassigned vehicle brake is acted upon by a low testing brake pressure.“Low” means that the testing brake pressure is significantly lower thanthe brake pressure during a full braking and, during normalstraight-ahead driving of the vehicle, results in no braking or only toan insignificant braking of the wheel or of the vehicle.

[0025] In Step 4, it is examined whether or how the rotational wheelspeed behavior of the wheel of the brake acted upon by testing brakepressure changes. If the rotational wheel speed does not change orchanges only insignificantly, this leads to the conclusion that asufficiently high normal wheel force is present at the considered wheelon the inside of the turn, which causes the wheel to rotate along, andthat therefore no overturn risk is present. In this case, Step 1 isrestarted. In contrast, if the wheel is braked intensively, thisindicates a condition 5 in which an overturn risk exists.

[0026] In this event, a control intervention by the braking system takesplace in Step 6. As a result of a possibly wheel-selective brakingintervention, the normal wheel force will increase again and theoverturn risk will be reduced. If the vehicle continues to be unbraked,Step 1 will be started again.

[0027]FIG. 2 is a flow chart for estimating the overturn risk of abraked vehicle. “Braked” means that the driver defines a brake pressureby way of the brake pedal, which brake pressure is higher than theabove-mentioned testing brake pressure. Corresponding to FIG. 1, thetransverse vehicle acceleration a_(trans) is determined in Step 1, andit is examined in Step 2 whether the sensed transverse vehicleacceleration a_(trans) is higher or lower than or equal to the definedtransverse acceleration limit value a_(trans max).

[0028] If the sensed transverse vehicle acceleration a_(trans) is higherthan the transverse acceleration limit value a_(trans max), in Step 7,the brake pressure defined by the driver is lowered for a short time atone or more wheels on the inside of the turn. In Step 8, the occurringchange of the rotational wheel speed is then monitored. If therotational wheel speed of the examined wheel or wheels on the inside ofthe turn changes only little, it can be concluded that no normal wheelforce or only a slight normal wheel force is still present; that is,that the assigned wheel is just about to be lifted off the road or hasalready lifted off and is therefore no longer accelerated. It shouldtherefore be assumed that an overturn risk exists in condition 5 whichis reduced by a control intervention of the braking system in Step 6.

[0029]FIG. 3 describes a method of estimating the overturn risk on thebasis of instantaneous movement parameters of the vehicle. In the caseof this method, the transverse vehicle acceleration a_(trans) as well asadditional parameters characterizing the directional control, such asthe time derivation of the transverse vehicle acceleration da_(trans)/dtand the period duration T of a rolling motion of the vehicle, aredetermined in Step 9.

[0030] When sensing the transverse vehicle acceleration a_(trans) bymeans of a transverse acceleration sensor, diverse signal peaks mayoccur in the course of the transverse acceleration signal which aregenerated, for example, by noise signals, vehicle vibrations or shocksbecause of road unevennesses. In Step 10, such “disturbance parameters”are filtered out of the signals determined in Step 9, which results inthe parameters a′_(trans), da′_(trans)/dt, T′ which are freed ofdisturbance parameters.

[0031] By means of defined motion equations of the vehicle, in Step 11,the expected rolling motion of the vehicle is precalculated for animminent short time span of, for example, 0.5 to 1.5 s. Diversevehicle-specific parameters can be entered into these motion equations,for example, the vehicle mass in the unloaded condition, the position ofthe center of gravity of the vehicle, spring/damping parameters of thevehicle, the momentary engine torque as well as “marginal conditions”,such as the road inclination, etc.

[0032] By means of the precalculated rolling motion, it can be estimatedwhether an overturn risk exists in Condition 12. Analogous to themethods explained in FIGS. 1 and 2, in the event of the existence of anoverturn risk, a control intervention (Step 6) of the braking systemtakes place which reduces the overturn risk.

[0033] Particularly the method described in FIG. 3 permits a timelyrecognition of dangerous driving-dynamic conditions, for example, in anarrowing turn, in the event of a multiple lane change, during asuddenly initiated longer-lasting steering movement, etc., which maylead to an overturn situation.

[0034] The methods described in FIGS. 1 to 3 can be carried outseparately. However, it is particularly advantageous for the methods tobe used redundantly, that is, jointly for the recognition of criticaldriving conditions.

1. Method of estimating an overturn risk of a vehicle, in which thetransverse acceleration of the vehicle is constantly determined, and asa function of the lateral acceleration, the rotational wheel speedbehavior of the vehicle wheels is monitored, characterized in that theoverturn risk is determined redundantly, specifically a) as a functionof whether the vehicle is braked or unbraked by monitoring therotational wheel speed behavior, and b) by precalculating the rollingmotion of the vehicle to be expected by using the transverseacceleration (a_(trans)).
 2. Method according to claim 1, characterizedin that the estimation of the overturn risk is carried out only when adefined transverse acceleration (a_(trans max)) is exceeded.
 3. Methodaccording to one of claims 1 or 2, characterized in that, in the case ofan unbraked vehicle, a testing brake pressure is applied to a vehiclebrake, which testing brake pressure is low with respect to the brakepressure of a full braking, and in that the existence of an overturnrisk is assumed when the testing brake pressure results in a strongbraking of the assigned wheel.
 4. Method according to one of claims 1 to3, characterized in that, in the case of a braked vehicle, the brakepressure of a vehicle brake is lowered, and in that the existence of anoverturn risk is assumed when the rotational wheel speed of the assignedwheel increases only a little or not at all.
 5. Method according to oneof claims 1 to 4, characterized in that the time variation of thetransverse acceleration (da_(trans)/dt) is used when precalculating therolling motion.
 6. Method according to one of claims 1 to 5,characterized in that the vibration period (T) or the frequency of theinstantaneous rolling motion of the vehicle is used when precalculatingthe rolling motion.
 7. Method according to one of claims 1 to 6,characterized in that, before the precalculation, disturbing influencesare filtered out of the sensed movement parameters of the vehicle usedfor this purpose.
 8. Method according to one of claims 1 to 7,characterized in that the expected rolling motion of the vehicle isprecalculated for 0.5 to 1.5 s.
 9. Method according to one of claims 1to 8, characterized in that, when an overturn risk is determined, abraking intervention of the braking system of the vehicle takes placewhich reduces the overturn risk.
 10. Method according to one of claims 1to 9, characterized in that, when an overturn risk is recognized, on thebasis of Steps a) and b) of claim 1, a braking intervention for thebraking system is determined in each case, and that braking interventionis carried out which has the higher braking demand.
 11. Method accordingto one of claims 1 to 10, characterized in that, in Step a), therotational wheel speed behavior of wheels on the inside of the turn ismonitored.
 12. Method according to one of claims 1 to 11, characterizedin that, in the case of an unbraked vehicle, the testing pressure isapplied to vehicle brakes of wheels on the inside of the turn. 13.Method according to one of claims 1 to 12, characterized in that, in thecase of a braked vehicle, the brake pressure is lowered at vehiclebrakes of wheels on the inside of the turn.